This invention relates to drying printed web material. In rotary printing machines, and in particular in rotogravure printing machines, a web is printed at different printing stations in sequence. Invariably, at each printing station, there is a different amount of ink printed onto the web. This may vary from complete coverage of the web to approximately 10% coverage thereof. It is, of course, important that the printing ink is dry before the printed web is allowed to pass from one printing station to the next.
To ensure that adequate drying is achieved, each printing station is provided with web drying apparatus whereby a printed portion of the web is dried before that portion is allowed to pass through to the next printing station.
Ink used in rotary printing machines comprises a mixture of colour pigments and a binder, together with a solvent which acts as a carrier for the ink. The solvent is of a volatile nature and a mixture of evaporated solvent with air presents an explosion hazard if the air/solvent volume ratios (v/v) are in the flammable range of the particular solvent or solvents being employed. Each solvent has its own flammable range. For example, a solvent known as TOLUENE has a range of 1.3% to 7% v/v, being quantified as 1.3% Lower Explosive Limit (L.E.L.) and 7% Upper Explosive Limit (U.E.L.). The U.E.L. does not enter into calculations as a web drying system should be designed on excess air volumes relative to the flammable range.
In the case of printing machines which do not incorporate continuous automatic sensing means for determining the concentration level of solvent vapour/air mixtures, it is a requirement in the United Kingdom by H.M. Factory Inspectorate that the mixture level must not exceed 25% of the L.E.L. of the particular solvent or solvents used. But in printing machine with continuous automatic sensing means, 45% of the L.E.L. is permitted provided that at 50% an audible warning is given automatically and at 55% the printing machine is automatically stopped.
Known apparatus for drying printed web material make use of heated air which is discharged on to the printed portion of web so as to dry it.
The L.E.L. is maintained at a permissible level by collecting the mixture of heating air and solvent vapour resulting from the web-drying operation, (hereafter the solvent mixture).